1. Field of the Invention
The present invention relates to an asynchronous handover (DHO: diversity handover) system between base stations for use in a radio telecommunication system.
2. Description of the Related Arts
FIG. 1 is a diagram describing the timing for transmitting and receiving data in a conventional handover system.
Referring to FIG. 1, the BTS (Base Transceiver Station) 1 is a base station wherein a mobile station MS is initially engaged in a telecommunication, and the SFN (System Frame Number) 1 and DPCCH (Dedicated Physical Control Channel) 1 show transmission timings of data transmitted from the BTS 1. The BTS 2 is a base station wherein the mobile station MS begins a telecommunication, and the BTS 2 transmits SFN 2 and Rounded (DL DPCCH 2) transmission timings of data. The DPCCH 1 and DPCCH 2, which are parenthesized as “MS”, show timings of a mobile station MS receiving respective pieces of data transmitted from a BTS 1 and a BTS 2.
The mobile station MS moves from the DHO origin base station BTS 1 to the DHO destination base station BTS 2. Numeral (1) shows a frame number specific to the BTS 1. Downlink specific control channels are transmitted when the results of the Frame Offset and Chip Offset are added to numeral (1). This signal reaches the mobile station after a propagation delay PD1, at the time indicated by numeral (3). The mobile station adjusts the window timing so that the path from a base station (i.e., BTS 1 according to FIG. 1), which has the largest reception power of the plurality of base stations, arrives at the center of a search window. The mobile station transmits an uplink signal at a time indicated by numeral (5), in a prescribed timing offset from the timing of numeral (4). Meanwhile, the BTS 1 opens a search window at the time indicated by numeral (6), in a prescribed timing offset (which is the same as the mobile station). This is done after the timing indicated by numeral (2), so as to be on standby for a mobile station's uplink signal path. The uplink signal transmitted at the aforementioned timing in numeral (5) is received at the base station at the time indicated by numeral (6) by way of a propagation delay PD1.
A mobile station keeps monitoring downlink signals from a plurality of base stations and, if a base station (or a sector) emerges having reception power that exceeds a threshold value, the mobile station starts a handover process.
In the following description, it is assumed that the reception level from the BTS 2 exceeds a reception power threshold value set at the mobile station, and starts a handover process. In this event, the mobile station calculates the BTS's 2 downlink transmission timings using the following method so that the path from the BTS 2 arrives at the center of the mobile station's search window.
The BTS 2 has a frame number shown in numeral (7). Since this is specific to an individual base station, it is asynchronous. The frame number is received by the mobile station, by way of a propagation delay, at the time indicated by numeral (8). The mobile station calculates the difference between the head of the BTS 2 frame number and the reception of the timing from a downlink specific ch (i.e., DPCCH1) with expression (9). The mobile station notifies an upper layer apparatus of this parameter, and the upper layer apparatus in turn notifies the BTS 2 of the timing difference reported by the mobile station. Incidentally, if the BTS 2 transmits the frame number at the time indicated by numeral (10), by applying a transmission timing that is reported from the upper layer apparatus, then the path arrives at the position in numeral (4). This enables the path to be captured in a limited search window, which is very convenient for the mobile station.
FIG. 1 is an example of rounding up to an earlier timing. Because the base station needs to maintain orthogonality of despreading codes between each transmission channel, it must round up a downlink transmission timing to a despreading code border without failing. In an asynchronous inter-base station handover, which is carried out for the code division multiple access (CDMA) system, the round up process is always required. As a result of the round up process, the transmission timing changes to numeral (11), and the uplink signal reception timing changes from numeral (12) to numeral (13) at the same time.
In case of a forward round up, there is a case of a mobile station's path timing (indicated by numeral (14)) running over the back of a reference reception timing, thus requiring a countermeasure. Inversely, in the case of a backward round up, the path timing numeral (14) may run over forward.
That is, when performing a handover (DHO) between radio telecommunication base stations in a radio telecommunication method, a mobile station's path may exceed a reference timing for reception (cell radius: the larger the cell radius, the larger the delay of reception data, thereby causing possible large differences in reception timings, and possible reception data timings running over the reception window) (hereinafter noted as “reception reference timing”) because the base station's frame numbers are asynchronous. Therefore, the base station is required to change or enlarge a reception reference timing appropriately in accordance with the arrival timing (i.e., path) of a signal from a mobile station, so as to track the operations.
Meanwhile, it is not possible for a base station to judge whether a mobile station is in a DHO state, or the DHO has been completed, because the mobile station does not send a completion report when the DHO is completed. Accordingly the base station is required to continue telecommunication in the DHO state forever, thus resulting in a waste of resources and processes, and an inability to measure an accurate propagation time to the mobile station.
Reference patent document 1 discloses a CDMA receiver capable of flexibly tracking a mobile station during a DHO without requiring a complicated equipment configuration.
However, in the conventional technique, there has been no invention related to a method for accomplishing a detail process when a DHO is between asynchronous radio base stations. For example, even with the contents of patent document 1 noting that “a reference reception timing is adjusted at the time of a DHO between asynchronous base stations”, there has been no invention to specifically describe how it is adjusted. Nor has there been any invention focusing on the completion of a DHO, thus requiring an extraneous process and resources upon completing a DHO, thereby causing inefficiency.    Patent document 1: Laid-Open Japanese Patent Application Publication No. 2001-111456